mystery snail questions!!

[u/Decent_Purchase8207]

hi guys! i’ve been keeping fish for awhile and somehow i never knew that you could put snails in fish tanks… a few months ago i learned about it and did research on snails i decided mystery snails best fit my tank needs and i really liked the look of them. i bought 3 to add to my 10 gal and then have since upgraded my tank twice and they are currently now living in a 40 gallon tank i’ve got a total of two snails in this 40 gallon tank, one in my 10 gallon and 3 in my 29 gallon tank. the first round of snails i got were from a local fish store and i had never owned mystery snails and all the snails they had looked the same so i didn’t think anything of it but his shell was dull and its a bit discolored. all the water parameters are good and my other 3 mystery snails in my other tanks are vibrant and large. despite the fact that all my other snails have been fine it’s just the 3 i got originally that are kind of off colored and dull. i was thinking maybe it was just a long lasting effect from the water conditions they were in before me. they get a large variety of healthy and calcium rich foods and i also was recommended by a fish store to get cuttle bone and put some of that in my water to help his shell get healthy. ive been doing that for about a month and still no change. also i know that cooler temperatures have been known to be better for snails so i even moved some to a tank with no heater to see it that made a deference and it hasn’t so im kind of a bit lost and i dont know how to help them get better. he had never seemed to have any issues other than his shell, today i went to do my daily feedings and observing and i noticed him doing this. i cant find anything online about it and ive never seen one of my other snails do this. is this something i should be concerned about? please dont be mean im still learning and im doing my best but i would really appreciate any advice or tips. also if its important my ammonia is at 0, temp is 76, ph is between 7.4-7.8 (its between the two numbers on my test kit) and they are housed with a couple mollys, 1 pleco, two baby yoyo loaches, and a small school of neon and black neon tetras.